Method for producing bonding wafer

ABSTRACT

A method for producing a bonded wafer by the ion implantation delamination method includes at least a step of bonding a bond wafer having a micro bubble layer formed by gaseous ion implantation and a base wafer serving as a support substrate and a step of delaminating the bond wafer at the micro bubble layer as a border to form a thin film on the base wafer. After the delamination of the bond wafer, the bonded wafer is subjected to a heat treatment in an atmosphere of an inert gas, hydrogen or a mixed gas thereof, then the bonded wafer is subjected to thermal oxidation to form a thermal oxide film on the surface of the thin film, and then the thermal oxide film is removed to reduce thickness of the thin film.

TECHNICAL FIELD

The present invention relates to a method for producing a bonded waferutilizing the ion implantation delamination method, in particular, amethod for producing an SOI wafer by bonding a silicon wafer implantedwith hydrogen ions or the like to another wafer serving as a supportsubstrate and then delaminating the wafer.

BACKGROUND ART

Recently, as a method for producing an SOI wafer, the method comprisingbonding a wafer implanted with hydrogen ions or the like and thendelaminating the wafer to produce an SOI wafer (a technique called ionimplantation delamination method: Smart Cut Method (registeredtrademark)) is newly coming to attract much attention. This method is atechnique for producing an SOI wafer, wherein an oxide film is formed onat least one of two silicon wafers, gaseous ions such as hydrogen ionsor rare gas ions are implanted into one wafer (bond wafer) from its topsurface to form a micro bubble layer (enclosed layer) in this siliconwafer, then the ion-implanted surface of the wafer is bonded to theother silicon wafer (base wafer) via the oxide film, thereafter thewafers were subjected to a heat treatment (delamination heat treatment)to delaminate one of the wafers (bond wafer) as a thin film at the microbubble layer as a cleavage plane, and the bonded wafer is furthersubjected to a heat treatment (bonding heat treatment) for firm bondingto obtain an SOI wafer (refer to Japanese Patent Laid-open (Kokai)Publication No. 5-211128). In this method, the cleavage plane(delaminated plane) is obtained as a good mirror surface, and an SOIwafer also having high uniformity of the thickness of the SOI layer iscomparatively easily obtained.

However, when an SOI wafer is produced by the ion implantationdelamination method, a damaged layer remains on the surface of SOI waferafter delamination due to the ion implantation, and the surface hashigher surface roughness compared with mirror surfaces of silicon wafersof usual product level. Therefore, it becomes necessary to remove such adamaged layer and roughening of the surface in the ion implantationdelamination method. Conventionally, in order to remove the damagedlayer and so forth, mirror polishing using a small amount of stockremoval for polishing (stock removal: about 100 nm), which is calledtouch polish, has been performed in a final step after the bonding heattreatment.

However, if the SOI layer is subjected to polishing involving amechanical processing factor, there is caused a problem that uniformityof the SOI layer thickness attained by implantation of hydrogen ions orthe like and the delamination is degraded, because the stock removal forpolishing is not uniform.

Therefore, Japanese Patent Laid-open (Kokai) Publication No. 10-242154proposed a method for improving the surface roughness by subjecting anSOI wafer obtained by the ion implantation delamination method to a heattreatment in an active atmosphere (hydrogen atmosphere) withoutpolishing the surface of the SOI wafer. It is described that, accordingto this method, the surface roughness of the SOI layer surface can beimproved while maintaining the uniformity of film thickness of the SOIlayer.

However, damages resulting from the ion implantation exist in an SOIwafer obtained by the ion implantation delamination method, and thedamages in the SOI layer are large at the surface side and becomesmaller at a deeper position in the layer. Therefore, if the wafer issubjected to such a heat treatment in an active atmosphere as describedabove, recovery of the damages advances from interior of the SOI layerto the surface side. However, when the damages at the surface side arelarge, a heat treatment at a high temperature for a long period of timeis required, and in addition, complete recovery may not be obtained evenif a heat treatment is performed at a high temperature for a long periodof time, as the case may be.

Since the strength and depth of the damages are influenced by the amountof implantation energy and dose of gaseous ions such as hydrogen ions,for example, when the implantation energy needs to be increased as inthe case of producing an SOI wafer having a thick SOI layer or a thickburied oxide layer, or when the dose needs to be increased for thepurpose of performing the delamination heat treatment at a lowtemperature, the aforementioned problem becomes remarkable.

Furthermore, if the wafer is subjected to a heat treatment at a hightemperature for a long period of time under a reducing atmospherecomprising hydrogen gas, silicon at the SOI layer surface may be etched,thus thickness uniformity may be degraded, and etch pits may be formedin a buried oxide layer. This phenomenon is caused by the reasondescribed below. That is, defects such as COPs (Crystal OriginatedParticles) exist in the SOI layer, and if they are connected to theoxide film as the under layer, COPs do not disappear and remain as theyare, or they are even enlarged. Therefore, hydrogen or the like thatpenetrates through the defects also etches the buried oxide layer, andthus pits are formed there to cause the aforementioned phenomenon. Theseetch pits are problematic, since they influence also on the SOI layerneighboring the pits.

As described above, although various methods have been proposed in orderto remove the damaged layer and roughening of the surface of SOI waferobtained by the ion implantation delamination method while maintainingthe thickness uniformity of SOI layer, there are no satisfactory methodso far, and a suitable solution method has been desired.

Therefore, as described in Japanese Patent Laid-open (Kokai) PublicationNo. 2000-124092, the applicants of the present invention proposed, as amethod for producing an SOI wafer of high quality by removing thedamaged layer and the roughening of the surface remaining on a surfaceof SOI layer after the delamination in the ion implantation delaminationmethod while maintaining the thickness uniformity of the SOI layer, amethod of forming an oxide film by a heat treatment under an oxidizingatmosphere on the SOI surface after delamination, then removing theoxide film, and subsequently subjecting the wafer to a heat treatmentunder a reducing atmosphere.

If the so-called sacrificial oxidation is performed, in which an oxidefilm is formed on the SOI layer by a heat treatment under an oxidizingatmosphere, and then the oxide film is removed, a part of or wholedamaged layer on the SOI layer surface can be incorporated into theoxide film, and therefore the damaged layer can efficiently be removedby removing the oxide film. Further, a heat treatment can besubsequently performed under a reducing atmosphere to recover thedamaged layer remaining in the SOI layer and improve the surfaceroughness, and the heat treatment time of the heat treatment under areducing atmosphere can also be shortened, since a part of or wholedamaged layer on the SOI layer surface has been removed by thesacrificial oxidation. Furthermore, this method does not requirepolishing or the like involving a mechanical processing factor,uniformity of the film thickness of the SOI layer is not degraded, andthus it is considered that an SOI wafer of extremely high quality can beproduced by the ion implantation delamination method with higherproductivity.

As described above, the technique described in Japanese PatentPublication (Kokai) Laid-open No. 2000-124092 has the advantage that thedamaged layer and the roughening of the surface remaining on the SOIlayer surface after the delamination can be removed in the ionimplantation delamination method while maintaining the thicknessuniformity of the SOI layer. However, when the inventors of the presentinvention performed additional experiments for this technique, it wasfound that the technique had the following drawbacks and thus thetechnique was insufficient as it was as a method for production in alarge scale.

-   1) Since the sacrificial oxidation in the aforementioned technique    directly oxidizes the delaminated plane obtained by the ion    implantation, oxidation induced stacking faults (OSFs) may be    generated by the oxidation, and these OSFs may not be completely    removed only by the subsequent heat treatment under a reducing    atmosphere.-   2) Surface roughness sufficiently removed by the heat treatment    under a reducing atmosphere mainly consists only of short period    components (e.g., period of 1 μm or less), and removal of long    period components (e.g., period of about 1 to 10 μm) of the surface    roughness may become insufficient.-   3) If the high temperature heat treatment is performed under an    atmosphere containing a large amount of hydrogen gas as the reducing    atmosphere, the hydrogen gas acts on the bonding interface, and thus    corrosion of the interface may become significant and thus it may    cause particle generation in the device production process.

DISCLOSURE OF THE INVENTION

The present invention was accomplished in order to solve theaforementioned problems, and its object is to provide a method forproducing a bonded wafer, which can surely remove damages or defects ona surface of a wafer produced by the ion implantation delaminationmethod and sufficiently flatten the surface roughness while maintainingthe thickness uniformity of the bonded wafer, and which can also besufficiently used as a technique for large scale production.

In order to achieve the aforementioned object, the present inventionprovides a method for producing a bonded wafer by the ion implantationdelamination method comprising at least a step of bonding a bond waferhaving a micro bubble layer formed by gaseous ion implantation and abase wafer serving as a support substrate and a step of delaminating thebond wafer at the micro bubble layer as a border to form a thin film onthe base wafer, wherein, after the delamination of the bond wafer, thebonded wafer is subjected to a heat treatment in an atmosphere of aninert gas, hydrogen or a mixed gas thereof, then the bonded wafer issubjected to thermal oxidation to form a thermal oxide film on thesurface of the thin film, and then the thermal oxide film is removed toreduce thickness of the thin film.

If the delaminated plane of the thin film after the delamination step issubjected to a heat treatment under an atmosphere of an inert gas,hydrogen gas or a mixed gas thereof to perform a surface flatteningtreatment and removal of damages, and then the oxidation and the removalof the oxide film are performed as described above, uniformity of thefilm thickness can be maintained, and generation of OSFs due to theoxidation can also surely be avoided.

In the aforementioned method, after the heat treatment under anatmosphere of an inert gas, hydrogen gas or a mixed gas thereof, thesurface of the thin film can be polished for a stock removal of 70 nm orless, and then the thermal oxidation can be performed.

If the surface is slightly polished (stock removal of 70 nm or less,especially 50 nm or less) after the heat treatment under an atmosphereof an inert gas, hydrogen gas or a mixed gas thereof, the long periodcomponents of surface roughness can be improved.

That is, although the short period components of surface roughness arefully removed by the heat treatment under an atmosphere of an inert gas,hydrogen gas or a mixed gas thereof, long period components may remain,and therefore they are removed by polishing. If the heat treatment isonce performed as described above, the surface roughness and damages ofthe surface are improved, and therefore the stock removal for polishingcan be reduced compared with that conventionally used, and inparticular, it may be reduced to a half or less of the conventionallyused stock removal. Thus, the long period components of surfaceroughness can surely be removed while the influence on uniformity offilm thickness is minimized.

Further, the heat treatment under an atmosphere of an inert gas,hydrogen gas or a mixed gas thereof is preferably performed under a 100%argon atmosphere or an argon atmosphere containing hydrogen in an amountbelow explosion limit.

Such an atmosphere enables improvement of surface roughness and damagesof the surface while suppressing corrosion of the bonding interface.

Further, in the present invention, a silicon single crystal wafer ispreferably used as the bond wafer.

If a silicon single crystal wafer, which enables production of wafershaving high quality and large diameter, is used as the bond wafer,gaseous ions are implanted into it, and it is delaminated, a bondedwafer having a silicon single crystal thin film of high quality and alarge diameter can be produced at a low cost.

Moreover, the bonded wafer produced by the method of the presentinvention can be a bonded wafer having a thin film of high qualityshowing high uniformity of film thickness and being free from rougheningof the surface and surface damages.

For example, there can be provided a bonded SOI wafer produced bybonding two of silicon wafers via an oxide film, wherein surfaceroughness (RMS) measured for a 1 μm square and 10 μm square of the SOIlayer surface is 0.15 nm or less for the both squares, and σ of thethickness of the SOI layer is 1.5 nm or less.

As explained above, according to the present invention, the damagedlayer and surface roughness remaining on the surface of the delaminatedthin film in the ion implantation delamination method can surely beremoved while maintaining the uniformity of film thickness of the thinfilm. Therefore, a bonded wafer of extremely high quality can beproduced with high productivity, and thus it is a method for producing abonded wafer extremely suitable as a technique for large scaleproduction.

BRIEF EXPLANATION OF THE DRAWING

FIGS. 1(a) to 1(i) show a flow diagram of an exemplary process forproducing an SOI wafer by the ion implantation delamination methodaccording to the present invention.

BEST MODE FOR CARRYING OUT THE INVENTION

Hereafter, embodiments of the present invention will be explained withreference to the appended drawings. However, the present invention isnot limited to these.

FIG. 1 shows a flow diagram of an exemplary process for producing an SOIwafer by the ion implantation delamination method according to thepresent invention.

The present invention will be explained hereafter by exemplifying a casewhere two of silicon wafers are bonded.

In the ion implantation delamination method shown in FIG. 1, twomirror-surface silicon wafers are prepared first in the step (a). Thatis, a base wafer 1 that serves as a support substrate and a bond wafer 2from which an SOI layer is obtained, which correspond to specificationsof devices, are prepared.

Then, in the step (b), at least one of the wafers, the bond wafer 2 inthis case, is subjected to thermal oxidation to form an oxide film 3having a thickness of about 0.1 to 2.0 μm on its surface.

In the step (c), gaseous ions such as hydrogen ions or rare gas ions,hydrogen ions in this case, are implanted into one surface of the bondwafer 2 on which the oxide film was formed to form a micro bubble layer(enclosed layer) 4 parallel to the surface in mean penetrating depth ofthe ions.

The step (d) is a step of superimposing the base wafer 1 on the hydrogenion implanted surface of the hydrogen ion implanted bond wafer 2 via anoxide film and bonding them. Usually, by contacting the surfaces of twoof the wafers to each other in a clean atmosphere at an ordinarytemperature, the wafers are adhered to each other without using anadhesive or the like.

The subsequent step (e) is a delamination heat treatment step in whichthe wafers were delaminated at the enclosed layer 4 as a border toseparate them into a delaminated wafer 5 and an SOI wafer 6 (SOI layer7+buried oxide layer 3+base wafer 1). For example, if the wafers aresubjected to a heat treatment at a temperature of about 400° C. to 600°C. under an inert gas atmosphere, the wafers are separated into thedelaminated wafer 5 and the SOI wafer 6 due to rearrangement of crystalsand aggregation of bubbles in the enclosed layer. On the SOI layer 7 atthe surface of the SOI wafer as it is after the delamination, a damagedlayer 8 remains.

Further, ions to be implanted such as hydrogen ions can be excited andimplanted in a state of plasma, or bonding surfaces can be preliminarilysubjected to a treatment with nitrogen, oxygen, hydrogen plasma or thelike so as to be activated and bonded, as the result the delaminationheat treatment can be omitted.

After this delamination step, the bonding heat treatment is performed inthe step (f). In this step, the SOI wafer 6 is subjected to a heattreatment at a high temperature as a bonding heat treatment to obtainsufficient bonding strength, since the bonding strength of the wafersbrought into close contact in the aforementioned bonding step and thedelamination heat treatment step in the steps (d) and (e) as it is wouldbe weak for use in the device production process. This heat treatment ispreferably performed, for example, under an atmosphere of inert gas at1000° C. to 1300° C. for 30 minutes to 2 hours. As for the steps thusfar, the method of the present invention is the same as the conventionalion implantation delamination method.

However, in the method of the present invention, this bonding heattreatment may be omitted, and the heat treatment in the following stepunder an atmosphere of an inert gas, hydrogen gas or a mixed gas thereofcan be used also as the bonding heat treatment (refer to FIG. 1, f′).Thus, the steps can be further simplified compared with the conventionalmethod of removing the roughening of the surface or surface damages.

Then, in the step (g), the SOI wafer after the bonding heat treatmentstep (f) (or after the delamination step (e) when the bonding heattreatment is omitted) is subjected to a heat treatment under an inertgas, hydrogen gas or a mixed gas thereof using a usual heat treatmentfurnace in which heating is attained by a heater (batch furnace) toimprove surface roughness and remove damages on the SOI surface. Theheat treatment temperature is suitably 1100° C. to 1350° C. When it islower than 1100° C., a long period of time is required to improve thesurface roughness. On the other hand, a temperature exceeding 1350° C.may cause a problem concerning contamination with heavy metal impuritiesor durability of the heat treatment furnace. Further, although the heattreatment time depends also on the heat treatment temperature, it issuitably in the range of 10 minutes to 8 hours. The heat treatment for aperiod shorter than that may provide insufficient improvement of thesurface roughness, and the heat treatment for a period longer than thatdecreases the productivity. When the aforementioned heat treatment isperformed by using an RTA (Rapid Thermal Annealing) apparatus, the heattreatment temperature is preferably 1200° C. or higher, and the heattreatment time is preferably 1 to 120 seconds. Moreover, these heattreatment using a batch furnace and heat treatment using an RTAapparatus can also be performed in combination.

The heat treatment atmosphere may consist of an inert gas, hydrogen gasor a mixed gas thereof. However, if the content of hydrogen gas is high,the aforementioned corrosion of the bonding interface becomes likely tobe generated, and slip dislocations become likely to be generated by theheat treatment. Therefore, the hydrogen gas content is preferably 25% orless. The hydrogen gas content is more preferably, lower than theexplosion limit (4%) in view of safety. Although argon gas, which ismost inexpensive and shows high versatility, is suitable as the inertgas, helium or the like may also be used.

As described above, in the present invention, a sacrificial oxide filmis not directly formed on the delaminated plane, but the surfaceroughness and the damages of the surface are improved by firstperforming the heat treatment of the step (g) without generating OSFs inthe thin film and without degrading the thickness uniformity.

Then, since damages on the SOI surface cannot be sufficiently removedonly by the heat treatment of the step (g) in many cases, damages on theSOI surface are taken into an oxide film by the thermal oxidation in thestep (h), and thickness of the thermal oxide film to be formed iscontrolled at the same time so that thickness of the obtained SOI layershould become a desired thickness. If the thickness of the SOI layer isreduced by the thermal oxidation, a thinner film can be obtainedsubstantially without degrading thickness uniformity.

Further, if the thermal oxide film is removed by using, for example, anaqueous solution containing HF in the step (i), an SOI wafer having anSOI layer of the desired thickness is formed.

Thus, an SOI wafer having an SOI layer with a desired thickness can beobtained, in which the roughening of the surface and damages of thesurface are surely removed while thickness uniformity is maintained

In addition, after the heat treatment in the step (g) and before thethermal oxidation step (h), a polishing step using a stock removal of 70nm or less, especially 50 nm or less, may be added as required (refer toFIG. 1, g′). By adding this polishing step, long period components ofthe surface roughness, which cannot be removed by the heat treatment ofthe step (g), can be surely removed. Further, since the stock removal islimited to 70 nm or less, especially 50 nm or less, it is smaller thanthe stock removal conventionally used (about 100 nm or more), and it canbe reduced to, for example, a half or less of the conventionally usedstock removal. Thus, degradation of thickness uniformity of the SOIlayer can be markedly suppressed.

Hereafter, explanations will be specifically made with reference toexamples of the present invention and comparative examples. However, thepresent invention is not limited to these.

EXAMPLES 1 TO 4 AND COMPARATIVE EXAMPLES 1 TO 3

A silicon single crystal ingot produced by the Czochralski method andhaving a crystal orientation of <100>, conductivity type of p-type and aresistivity of 20 Ω·cm was sliced and processed to prepare mirrorsurface silicon wafers having a diameter of 200 mm. These were dividedinto bond wafers and base wafers, and SOI wafers were produced by theion implantation delamination method of the present invention accordingto the steps represented in FIGS. 1, (a) to (i).

First, according to FIGS. 1(a) to (e), a bond wafer 2 was delaminated toobtain an SOI wafer 6. At this time, thickness of a buried oxide layer 3was 400 nm, and the other major conditions such as those for ionimplantation were as follows.

-   1) Ion implantation conditions: H⁺ ions, implantation energy: 90    keV, implantation dose: 6.5×10¹⁶/cm²-   2) Delamination heat treatment conditions: under N₂ gas atmosphere,    500° C., 30 minutes

Thus, the SOI wafer 6 having an SOI layer 7 with a thickness of 437 nmcould be obtained. When surface roughness of the surface (delaminatedsurface) of the SOI wafer 6 as delaminated shown in FIG. 1(e) wasmeasured for a 1 μm square by the atomic force microscope method, it wasfound to be 6.7 nm in average in terms of the RMS value(root-mean-square roughness value). This value is 10 times or more of avalue for surface roughness of usual mirror-polished silicon singlecrystal wafers, and it can be seen that the surface of the SOI layer asdelaminated has significant local roughening of the surface (shortperiod components). Moreover, the RMS value for a 10 μm square, whichserves as an index of long period components, was also large, i.e., aslarge as 5.5 nm in average.

Each SOI wafer immediately after the delamination steps (e) was used andprocessed according to the processing flows shown in Tables 1 and 2.Surface roughness (RMS), thickness uniformity (average: t, standarddeviation: σ) and defect density of the SOI layer of each obtainedbonded wafer were measured. Treatment conditions and measurementconditions are shown in Table 3.

TABLE 1 Comparative Comparative Example 1 Example 2 Example 1 Example 2Processing No bonding Bonding heat No bonding Bonding heat flow heattreatment heat treatment treatment ↓ treatment ↓ ↓ Ar annealing ↓Sacrificial Ar annealing ↓ Ar annealing oxidation ↓ Polishing ↓Sacrificial for 40 nm Removal of oxidation ↓ oxide film ↓ Sacrificial ↓Removal of oxidation Ar annealing oxide film ↓ Removal of oxide film

TABLE 2 Comparative Example 3 Example 4 Example 3 Processing Bondingheat Bonding heat Bonding heat flow treatment treatment treatment ↓ ↓ ↓Ar annealing Ar annealing Polishing for ↓ ↓ 100 nm Polishing forPolishing for 55 nm 70 nm ↓ ↓ Sacrificial Sacrificial oxidationoxidation ↓ ↓ Removal of Removal of oxide film oxide film

TABLE 3 Treatment conditions and measurement conditions (Bonding heattreatment) 1100° C., 120 minutes (100% N₂ atmosphere) (Ar annealing)1200° C., 60 minutes (100% Ar atmosphere) (Sacrificial oxidation) 950°C., pyrogenic oxidation, oxide film thickness: 590 nm (Example 1), 500nm (Example 2) , 465 nm (Example 3) , 435 nm (Example 4) , 590 nm(Comparative Example 2) (Removal of oxide film) Etching with 5%hydrofluoric acid (Surface roughness measurement) AFM produced by Veeco,RMS measurement for 1 μm square and 10 μm square (Thickness distributionmeasurement) AcuMap 2 produced by ADE, measured for 1765 points in aplane (Defect measurement) Diluted Secco etching until SOI layerthickness becomes 30 nm → Etching with hydrofluoric acid → Observationwith optical microscope

Hereafter, the defect measurement method for the SOI layer will beexplained briefly.

In a case of thin film SOI as in the present invention, if the Seccoetching solution (mixture of dichromic acid, hydrofluoric acid andwater), which is used for the preferential etching of usual siliconwafers, is used, the etching rate becomes unduly high, and the SOI layeris removed by the etching for a short period of time. Therefore, it isnot suitable for evaluation of defects.

Therefore, the etching is performed by using the Secco etching solutiondiluted with pure water so as to reduce the etching rate until thethickness of the SOI layer becomes a predetermined thickness. By thisetching, defect portions in the SOI layer become micro pits penetratingthe SOI layer. Since it is difficult to observe these micro pits as theyare, the buried oxide layer is etched through the micro pits byimmersing the wafer in hydrofluoric acid to visualize the defectportions. Thus, the defect portions can be easily observed from thesurface of the thin film SOI layer by using a optical microscope.

In these examples and comparative examples, a commercially availableusual Secco etching solution was diluted twice and used, and the dilutedSecco etching was terminated when the thickness of the remaining SOIlayer became about 30 nm. Then, the wafer was immersed in 25 weight %hydrofluoric acid for 90 seconds, and defect density was measured byobserving the pits formed in the buried oxide layer by using a opticalmicroscope of 100 magnifications.

The measurement results are shown in Tables 4 and 5.

TABLE 4 Thickness RMS (nm) distribution Defect 1 μm 10 μm (nm) densitysquare square Average t σ (number/cm²) Example 1 0.08 0.29 171.5 0.2 1 ×10² Example 2 0.06 0.16 172.4 1.0 1 × 10² Comparative 0.10 0.28 437.00.2 5 × 10⁵ Example 1 Comparative 0.11 0.30 171.5 0.2 2 × 10⁴ Example 2

TABLE 5 Thickness RMS (nm) distribution Defect 1 μm 10 μm (nm) densitysquare square Average t σ (number/cm²) Example 3 0.08 0.15 172.5 1.4 1 ×10² Example 4 0.09 0.14 171.5 1.5 1 × 10² Comparative 0.10 0.13 437.02.0 5 × 10⁵ Example 3

From the results shown in Tables 4 and 5, it can be seen that, forsufficiently removing roughening of the surface and damages on thesurface resulting from ion implantation and forming an SOI layer havingfew defects, the production method of the present invention used forExamples 1 to 4 is suitable. Moreover, in Example 1, removal of adelaminated surface by polishing is not performed at all, and thereforeextremely superior thickness uniformity was obtained. Long periodcomponents of the surface roughness was improved by 10 times or morecompared with those immediately after the delamination, although theyare slightly inferior compared with Examples 2 to 4 where polishing wasperformed for 70 nm or less. Moreover, in Examples 2 to 4, although alittle degradation of the thickness uniformity was observed due to theinfluence of polishing, σ was still maintained at a high level of 1.0 to1.5 nm. Furthermore, extremely high quality was obtained for surfaceroughness for both of the short period components and long periodcomponents.

On the other hand, it can be seen that, if the Ar annealing alone wasused as in Comparative Example 1, the defect density becomes markedlyhigh, and surface damages cannot be completely removed. Further, it canbe seen that, as in Comparative Example 2, although the effect of defectreduction was obtained by performing the sacrificial oxidation beforethe Ar annealing, the defect density became higher compared withExamples 1 to 4 due to the generation of OSFs in the oxidation step.Moreover, if the polishing alone was used as in Comparative Example 3,the defect density became markedly high, and in addition, the thicknessuniformity was bad because the polishing was performed for no less than100 nm.

Thus, it can be seen that, according to the present invention, there canbe produced, for example, a bonded SOI wherein surface roughness (RMS)measured for a 1 μm square and 10 μm square of the SOI layer surface is0.15 nm or less for the both squares, σ of the thickness of the SOIlayer is 1.5 nm or less, and moreover defect density is less than 10³number/cm².

Furthermore, although 100% Ar atmosphere was used as the atmosphere forthe heat treatment for improving the surface roughness in theaforementioned examples, when the experiments were performed by using anargon atmosphere containing 3% hydrogen gas instead of the aboveatmosphere, results substantially similar to those mentioned above wereobtained. Further, in the heat treatment under 100% Ar atmosphere inExamples 1 to 4 and the heat treatment utilizing the argon atmospherecontaining 3% hydrogen gas instead of that, such corrosion of thebonding interface as seen in a heat treatment under an atmospherecontaining a large amount of hydrogen such as 100% hydrogen atmospherewas not observed.

The present invention is not limited to the embodiments described above.The above-described embodiments are mere examples, and those having thesubstantially same configuration as that described in the appendedclaims and providing the similar functions and advantages are includedin the scope of the present invention.

For example, while the present invention was explained above mainly forthe cases of bonding two of silicon wafers to produce SOI wafers, thepresent invention is not limited to those, and of course it can also beapplied to a case where an ion-implanted silicon wafer is bonded to aninsulator wafer and delaminated to produce an SOI wafer, and a casewhere a compound semiconductor wafer such as GaAs wafer is used as awafer to be implanted with ions.

Further, the production steps for producing the bonded wafer accordingto the present invention are not limited to those mentioned in FIG. 1.Other steps such as cleaning and heat treatment may be added, and thesteps are used with suitable modification including alteration of theorder of the steps, omission of some steps and so forth depending on thepurpose.

1. A method for producing a bonded wafer by an ion implantationdelamination method comprising at least a step of bonding a bond waferhaving a micro bubble layer formed by gaseous ion implantation and abase wafer serving as a support substrate and a step of delaminating thebond wafer at the micro bubble layer as a border to form a thin film onthe base wafer, wherein, after delaminating the bond wafer, the bondedwafer is subjected to a heat treatment in an atmosphere of an inert gas,hydrogen or a mixed gas thereof, then the bonded wafer is subjected tothermal oxidation to form a thermal oxide film on a surface of the thinfilm, and then the thermal oxide film is removed to reduce thickness ofthe thin film.
 2. The method for producing a bonded wafer according toclaim 1, wherein, after the heat treatment in an atmosphere of an inertgas, hydrogen or a mixed gas thereof, the surface of the thin film ispolished for a stock removal of 70 nm or less, and then the thermaloxidation is performed.
 3. The method for producing a bonded waferaccording to claim 1, wherein the heat treatment in an atmosphere of aninert gas, hydrogen or a mixed gas thereof is performed in a 100% argonatmosphere or an argon atmosphere containing hydrogen in an amount belowexplosion limit.
 4. The method for producing a bonded wafer according toclaim 2, wherein the heat treatment in an atmosphere of an inert gas,hydrogen or a mixed gas thereof is performed in a 100% argon atmosphereor an argon atmosphere containing hydrogen in an amount below explosionlimit.
 5. The method for producing a bonded wafer according to claim 1,wherein a silicon single crystal wafer is used as the bond wafer.
 6. Themethod for producing a bonded wafer according to claim 2, wherein asilicon single crystal wafer is used as the bond wafer.
 7. The methodfor producing a bonded wafer according to claim 3, wherein a siliconsingle crystal wafer is used as the bond wafer.
 8. The method forproducing a bonded wafer according to claim 4, wherein a silicon singlecrystal wafer is used as the bond wafer.
 9. A bonded wafer produced bythe method according to claim
 1. 10. A bonded wafer produced by themethod according to claim
 2. 11. A bonded wafer produced by the methodaccording to claim
 3. 12. A bonded wafer produced by the methodaccording to claim
 4. 13. A bonded wafer produced by the methodaccording to claim
 5. 14. A bonded wafer produced by the methodaccording to claim
 6. 15. A bonded wafer produced by the methodaccording to claim
 7. 16. A bonded wafer produced by the methodaccording to claim
 8. 17. A bonded SOI wafer produced by bonding twosilicon wafers via an oxide film, wherein surface roughness (RMS)measured for a 1 μm square and 10 μm square of an SOI layer surface is0.15 nm or less for both squares, and σ of a thickness of an SOI layeris 1.5 nm or less.